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g Students should be able to calculate the %by mass of an
element in a compound.
Fertilisers are usefulexamples for this type ofcalculation.
net.co.uk/page10/page10.htmClick on ChemicalCalculations
h calculate empirical and molecular formulae Suggested
experiment:Heating magnesium to form magnesiumoxide and working out
its formula.
Students should understand, with
reference to structures met in Units 2 and4, such as polyethene,
sodium chloride andsilicon dioxide, that some formulaerepresent
simple ratios. Suitablemolecules for illustrating empirical
formulaeinclude hydrocarbons (alkanes andalkenes), phosphorus
oxides and polymers.
See Appendix 1 of thecurrent syllabus for adviceabout
mathematicalrequirements. Thisexperiment is useful in terms
of processing a class set ofresults using a spreadsheetsuch as
Excel and plottingthe graph.
www.chemsoc.org/networks/learnnet/classic_exp.htmLook at
experiments 61, 67,90
i calculate reacting masses and volumes ofgases using the mole
concept
Students should practise working outreacting masses on both an
experimental
scale, using grams, and an industrial scale,using tonnes.
Less able students can work out reactingmasses from ratios of
masses in theequation without learning about molaramounts.
Students can be given a set of rules for
guidance to carry out these calculations.These can be stuck in
the back cover oftheir books for reference or in aprominent section
of their files - during laterUnits.
These skills should bepractised in the context of the
later units of the course. It iscommon for examinationquestions
to test calculationskills in the context of othersyllabus areas.
Suitablecontexts for practisingcalculations include reactionrates,
acids, bases and salts,metal reactions and
extraction and organicchemistry.
www.chemsoc.org/networks/l
earnnet/classic_exp.htmLook at experiment 68
www.wpbschoolhouse.btinternet.co.uk/page10/page10.htmClick on
ChemicalCalculations
http://www.boc.com/education/formulae_/formula.html
j process results of volumetric experimentsusing the concept of
solution concentration
Suggested experiments: Acid-base titrations e.g. dilute
hydrochloric or sulphuric acid withaqueous sodium hydroxide.
Potassium manganate(VII) titrations.
Use past practical (oralternative to practical)papers for
examples ofcommon volumetric
calculations.
www.chemsoc.org/networks/learnnet/classic_exp.htmLook at
experiments 8, 45,
48, 5
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Students should practise calculatingconcentrations from titres
and equations.
It is important that thetechniques of rough andaccurate titres
are practisedto prepare for the practicalpaper.
k calculate percentage yield and purity. Students need to be
able to identify thelimiting factor that determines themaximum
yield.Suggested experiments:
Determination of percentage yield:Preparation of copper sulphate
from anexcess of copper carbonate and a knownvolume of sulphuric
acid.
Purity: Determination of the coppercarbonate content of a
weighed sample ofmalachite by reaction with sulphuric acid.The
calculation can either be based on the
dried residue waste, or on a back-calculation from the mass of
coppersulphate produced (this will need heatingand drying to the
anhydrous form).
Again, use past practicalpapers for examples of thistype of
calculation.
Alternatively, this can betaught as an integral part ofthe
preparation of salts inUnit 8 or the amount ofmetals in ores in
Unit 6.
